EAF steel producers use electrical energy to melt raw materials to produce 1 ton to 420 metric tons of steel in vessels. Electrical energy can be delivered to the furnace as alternating current (AC) or direct current (DC). The electrical power delivered to the raw materials can be as high as 200 MWh in the case of the largest EAF vessels. This power supply creates an electrical arc that creates the necessary heat to raise the batch of steel to temperatures as high as 1800° C. and to allow for further refinement and processing in the LMF and subsequent casting and forming operations.
The electrical power is delivered to the steel through graphite electrodes. Graphite is the material of choice for electrodes due to the following characteristics: low coefficient of thermal expansion (CTE), high tensile strength, high specific resistance, electrical resistance that is relatively independent of temperature, and nobility (cathodic to other materials).
Electrodes are consumables utilized in the electrical steel making process and account for a substantial cost for the steel maker. The environment in the electric arc furnace is violent and harsh, and causes consumption of electrodes in a range of approximately 1 kg/metric ton of steel produced to 2.5 kg/metric ton. Causes of consumption include: electrical arc at the electrode tip where localized temperature is approximately 3000° C.; electrode breakage due to movement of raw materials; thermal shock and subsequent loss of electrode tip; and oxidation of the electrode surfaces along the column due to the harsh furnace environment. Oxidation of the electrode creates the conical shape of electrodes that are in use and can account for nearly 50% of the electrode consumption.
For decades, steel producers and furnace electrode producers have attempted to reduce the oxidation rate of the graphite and carbon electrodes through many different means. One example is to use electrodes that have surfaces coated with layers formed from graphite, metal, aluminum alloys, and pure aluminum. However, these coatings are only applied once (e.g., only during the manufacturing of the electrodes), and the coatings are susceptible to chemical and physical damage that renders them ineffective. Thus, these type of coatings can have short useful life spans.
Changes in the electrode manufacturing process, in electrode coupling technology, in the recipe for the graphite electrodes, and in operational procedures like foamy slag have substantially reduced electrode consumption since 1985 when electrode consumption was between 5 to 6 kg/metric ton of steel, to 1 to 2.5 kg/metric ton of steel in 2018. While this has been an impressive reduction, market forces have heightened sensitivity to the consumption rate. Even incremental decreases in consumption rate have a substantial impact to the steel maker.
The oxidation of the electrode is a chemical reaction. The rate of oxidation of the electrode increases with increasing temperatures because the reactant molecules have more kinetic energy at higher temperatures. The reaction rate (i.e., oxidation rate) is governed by the Arrhenius equation which in almost all cases shows an exponential increase in the rate of reaction as a function of temperature.
  k  =            -      Ea                      k        B            ⁢      T      
Where: k=the rate constant
kB=Boltzmann constant
T=absolute temperature
A=a constant for each chemical reaction
Ea=the activation energy
R=the universal gas constant
Therefore, many designs have been developed to cool the bulk of the electrode (i.e., lower the temperature of the electrode), but have been abandoned due to safety concerns. Applying cooling water to the electrode below the molten steel bath creates a very dangerous condition in the case of an electrode break or the failure of the cooling water channel. The release of cooling water below the steel bath creates an explosion due to the rapid expansion as the water changes phase from water to steam with an approximate volumetric expansion of 1,100 times. Electrodes used in commercial steel making are currently composed exclusively of graphite and do not contain cooling water channels.
To further reduce oxidation of the electrode, spray cooling was introduced to the industry and specific designs to cool the electrode using circular spray headers with multiple vertical spray headers located at multiple locations around the circumference of the electrode.
Investigation of water application has been employed to enhance safety as well as mitigate oxidation of the electrode. Enhancements, such as providing air to atomize the water as it is discharged from the spray nozzle, have been evaluated. Electrode cooling water flow, in some facilities, varies depending upon the furnace conditions, providing an additional level of safety.